Process for purifying 4,4&#39;-dihydroxydiphenylsulfone

ABSTRACT

The invention provides a process for purifying crude 4,4&#39;-dihydroxydiphenylsulfone characterized by dissolving the crude 4,4&#39;-dihydroxydiphenylsulfone in an aqueous solution of a basic substance, precipitating a mono-salt of 4,4&#39;-dihydroxydiphenylsulfone from the resulting solution by salting out, separating the salt off and subsequently treating the salt with an acid, and also porovides a process for preparing a mono-salt of 4,4&#39;-dihydroxydiphenylsulfone characterized by dissolving crude 4,4&#39;-dihydroxydiphenylsulfone in an aqueous solution of a basic substance, precipitating a mono-salt of 4,4&#39;-dihydroxydiphenylsulfone from the resulting solution by salting out, and separating the salt off.

TECHNICAL FIELD

The present invention relates to a novel process for purifying crude4,4'-dihydroxydiphenylsulfone to obtain 4,4'-dihydroxydiphenylsulfone ofhigh purity in a high yield, and to a novel process for preparing anintermediate of purification by the purifying process.

BACKGROUND ART

4,4'-Dihydroxydiphenylsulfone (hereinafter referred to as"4,4'-compound") is excellent in resistance to heat and oxidation,stability to light, etc. and has therefore found wide use in recentyears in the field of the high polymer industry relating to polyesterresin, epoxy resin, polycarbonate resin, polyether sulfone resin and thelike. When high-polymer products are prepared from 4,4'-compoundcontaining impurities such as 2,4'-dihydroxydiphenylsulfone (hereinafterreferred to as "2,4'-compound") and trihydroxytriphenyldisulfone(hereinafter referred to as "tri-compound"), the products tend to have adecreased molecular weight and impaired mechanical properties. Thistendency becomes more pronounced when 2,4'-compound, tri-compound andlike impurities are present in larger quantities. Accordingly, it hasbeen desired to supply 4,4'-compound of high purity free from theimpurities such as 2,4'-compound and tri-compound to the greatestpossible extent 4,4'-compound is useful also as a material for couplersfor color photography, as a color developer for heat-sensitive recordingpaper, etc., and a high purity is also desirable in such uses.

Industrially, 4,4'-compound is prepared primarily by the dehydrationreaction of phenol with a sulfonating agent such as sulfuric acid. Thereaction mixture contains 2,4'-compound, tri-compound and like sulfones,other sulfonic acids, etc. as impurities. When sulfonic acids, etc. areremoved from the reaction mixture, the resulting crude 4,4'-compoundusually contains about 70 to about 80 wt. % of 4,4'-compound.

Processes have been developed for preparing 4,4'-compound of high purityby inhibiting formation of 2,4'-compound. For example, a process forproducing 4,4'-compound of high purity is proposed wherein 4,4'-compoundis produced by a dehydration reaction while effecting precipitation ofthe product and isomerization of 2,4'-compound, a by-product, to4,4'-compound (U.S. Pat. No. 4,162,270). The crude 4,4'-compoundobtained by removing sulfonic acids and the like from the reactionmixture of this process usually contains about 90 to about 95 wt. %.Nevertheless, it is desired that the 4,4'-compound for the various usesmentioned above be at least about 97 wt. % in purity, for example,because higher qualities are required of resins in recent years.Accordingly, the compound prepared by the former process must be furtherpurified, and the compound obtained by the latter process shouldpreferably be further purified.

Processes for purifying 4,4'-compound have been proposed which employvarious organic solvent mixtures, such as o-dichlorobenzene-phenol(Examined Japanese Patent Publication SHO 51-36264),o-dichlorobenzene-acetic acid (Examined Japanese Patent Publication SHO57-48152), o-dichlorobenzene-ethyl acetate (Examined Japanese PatentPublication SHO 57-48153) and o-dichlorobenzene-alcohol (ExaminedJapanese Patent Publication SHO 58-2234). However, these processes alluse organic solvents and accordingly have the problem that the solventsare inconvenient to handle and hazardous from the viewpoint ofsanitation and cause environmental pollution.

Alternatively without using organic solvents, 4,4'-compound is usuallypurified by dissolving crude 4,4'-compound in an aqueous solution ofsodium hydroxide or like alkali, treating the solution with activecarbon and thereafter adding an acid, such as sulfuric acid, to thesolution in an amount at least sufficient to neutralize the solution toprecipitate 4,4'-compound This process utilizes the properties of4,4'-compound that it is sparingly soluble in water but readily solublein aqueous solutions of basic substances in the form of a mono- ordi-metal salt of the compound. This process removes sulfonic acids butalmost fails to remove 2,4'-compound, tri-compound and like sulfones,giving a product of a low purity which is in no way as high as isdesired as will be apparent from the comparative example given later.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a novel purifying process forproducing 4,4'-compound of high purity in a high purification yield(yield based on the content of 4,4'-compound in the crude) without usingorganic solvents in order to fully fulfill the foregoing requirement.

Another object of the invention is to provide a novel process forproducing a purification intermediate in a high yield by the aboveprocess for purifying 4,4'-compound.

These and other objects of the invention will become apparent from thefollowing description.

The present invention provides a process for purifying crude4,4'-compound characterized by dissolving the crude 4,4'-compound in anaqueous solution of a basic substance, precipitating a mono-salt of4,4'-compound from the resulting solution by salting out, separating thesalt off and subsequently treating the salt with an acid.

The invention further provides a process for preparing a mono-salt of4,4'-compound characterized by dissolving crude 4,4'-compound in anaqueous solution of a basic substance, precipitating a mono-salt of4,4'-compound from the resulting solution by salting out, and separatingthe salt off.

To fulfill the above objects, we have conducted intensive research andconsequently obtained the unique finding that when crude 4,4'-compoundis dissolved in an aqueous solution of a basic substance and thereaftersubjected to salting out, a mono-metal salt, mono-ammonium salt or likemono-salt of 4,4'-compound corresponding to the basic substance used canbe obtained with a high purity. We have further found that the salt,when subsequently treated with an acid, gives 4,4'-compound having ahigh purity of at least 99 wt. % usually in a high purification yield ofat least about 85%. The fact that 4,4'-compound can be salted out as amono-salt of high purity under a specified condition is totallyunexpected by one skilled in the art resorting to the common knowledgethat mono- or di-metal salts or mono- or diammonium salt of4,4'-compound is readily soluble in water. The present invention hasbeen accomplished based on the above novel findings.

The term "mono-metal salt" as used herein refers to a mono-alkali metalsalt, mono 1/2 alkaline earth metal salt or the like.

The processes of the invention are usable for purifying crudes of4,4'-compound of any purity including reaction mixtures for preparing4,4'-compound, such mixtures free from sulfonic acids, etc., andcommercial products of 4,4'-compounds.

According to the present invention, crude 4,4'-compound is firstdissolved in an aqueous solution of a basic substance usually withstirring. Examples of useful basic substances are hydroxides, carbonatesor the like of alkali metals such as sodium and potassium, and alkalineearth metals such as magnesium and calcium and ammonia. Especiallypreferable among these are sodium hydroxide, sodium carbonate andammonia.

The concentration of the aqueous solution of basic substance is notlimited specifically but is determined suitably in accordance with theamounts of basic substance and water to be used. To be suitable, theamount of basic substance to be used is the amount needed to neutralizethe sulfonic acids and like strong acids present, plus approximately atleast one equivalent to not greater than 2 equivalents per mole ofsulfones such as 4,4'-compound, 2,4'-compound and tri-compound. When theamount is less than the above range, free 4,4'-compound becomes mixedwith the mono-metal salt or monoammonium salt of 4,4'-compound saltedout to give crystals of impaired form and result in a lower effect toremove the impurities, hence objectionable. When a salting-out agent isadded directly to the solution of the crude compound containing thebasic substance in an amount exceeding the above range, a di-metal saltor diammonium salt of 4,4'-compound which is not salted out is formed ina large amount to result in an undesirable tendency toward a loweryield. However, use of more than two equivalents of the basic substanceper mole of 4,4'-compound and like sulfones is not substantially in anyway objectionable but is an advantageous mode of practicing theinvention. More specifically, when more than 2 equivalents of the basicsubstance is used excessively to obtain a solution containing a largeamount of di-metal salt or diammonium salt of 4,4'-compound each formedtherein and incapable of being salted out, the solution can be treatedfavorably with active carbon or the like when so required. An acid isthen added to the solution to convert the di-salt to a mono-salt and toform a salting-out agent at the same time, followed by further additionof salting-out agent when required, whereby the mono-salt can be saltedout.

When the crude 4,4'-compound is added to the aqueous solution obtainedby the dissolving step, the resulting liquid is in the form of asolution or suspension depending on the amount of the basic substance orwater. In the case of the suspension, the liquid phase generallycontains a mono-metal salt or monoammonium salt of 4,4'compound and asmall amount of di-metal salt or diammonium salt of 4,4'-compounddissolved therein, and the solid phase is composed predominantly of themono-salt of 4,4'-compound separating out upon oversaturation of theaqueous solution after dissolving the 4,4'-compound therein. Presumably,the mono-salt of 4,4'-compound further separates out from the liquidphase through salting out to give grown crystals.

Although the amount of water to be used for the dissolving step isvariable widely, it is suitable to use usually about 1.0 to about 8.0times the amount by weight of the crude 4,4'-compound. If the amount ofwater is smaller than this range, a suspension will be formed whichcontains a large proportion of solid phase to exhibit low fluidity andtends to become difficult to stir, whereas a larger amount of water, ifused, results in a tendency toward a decreased purification yield. Thus,amounts outside the range are not desirable. It is especially desirableto use water in about 1.3 to about 3.0 times the amount by weight of thecrude 4,4'-compound.

The temperature for the dissolving step, although not limitedspecifically, is favorably about 40° C. to the boiling point since themono-metal salt or monoammonium salt of 4,4'-compound can then be formedat an increased velocity.

The solution or suspension obtained above by treating the crude4,4'-compound is subjected to a salting-out step usually with stirringto precipitate the mono-metal salt or monoammonium salt of4,4'-compound. This step precipitates the mono-metal salt ormonoammonium salt of 4,4'-compound usually with a high purity of about98 wt. % or more in a high yield of about 86% or more based on the4,4'-compound in the crude. We have found this fact for the first time.

The salting-out step is executed by adding a salting-out agent to thesolution or suspension, or using an excess of the basic substance andadding hydrochloric acid, sulfuric acid or like acid to the solution orsuspension to form a salting-out agent, or by the combination of thesemethods. Examples of preferred salting-out agents are chlorides,sulfates or the like of alkali metals, such as sodium and potassium, andalkaline earth metals such as calcium and magnesium, and ammonium saltssuch as ammonium chloride and ammonium sulfate From the viewpoint ofpurification yield, purity, etc., it is desirable to use a salting-outagent which is a salt corresponding to the basic substance used. Theamount of salting-out agent to be used is variable over a wide range.Generally, it is suitable to use the agent in an amount not less than 2wt. % to the saturation concentration in terms of concentration. If theamount is smaller than this range, a lower salting-out effect and areduced yield will result, whereas amounts exceeding the saturationconcentration permit crystals of the salting-out agent to separate out.Amounts outside the above range are therefore undesirable. It is usuallydesirable to use the salting-out agent in an amount of about 4 to about15 wt. %.

The temperature at which the salting-out agent is to be added to thesolution or suspension, although not limited specifically, is favorablyabout 40° C. to the boiling point, since the mono-metal salt ormonoammonium salt of 4,4'-compound can be obtained in an improved formof crystals, with an improved purity and in a higher yield. Afteraddition of the salting-out agent, the mixture may be aged at thistemperature for about 0.2 to about 12 hours.

The mono-metal salt or monoammonium salt of 4,4'-compound precipitatedis then separated off as by filtration and suitably washed as requiredto give the mono-salt of 4,4'-compound having a high purity. Usually,the salt is subsequently treated with an acid to obtain free4,4'-compound. The acid treatment is conducted in the usual manner,i.e., by dissolving the mono-metal salt or monoammonium salt of4,4'-compound in water or an aqueous alkali solution, treating thesolution with active carbon when required, then adjusting the pH toabout 3 to about 6 with sulfuric acid, hydrochloric acid or like acid toprecipitate 4,4'-compound and separating off the precipitate as byfiltration. This method affords the compound substantiallyquantitatively.

In this way, the desired 4,4'-compound can be obtained with a highpurity in a high purification yield.

The high-purity mono-metal salt or monoammonium salt of 4,4'-compoundprepared by the above process can be used as it is as an industrialmaterial depending on the use.

The processes of the invention have the outstanding advantages givenbelow.

(1) The process wherein no organic solvent is used can be practiced by afacilitated procedure and poses no problem in respect of sanitation andenvironment.

(2) The process usually affords 4,4'-compound with a high purity of atleast 99 wt. % in a high purification yield of at least 85%, thus fullysatisfying the demand of the industry.

(3) The process purifies a larger amount of crude with use of a reducedamount of liquid more efficiently than the conventional processes andcan be practiced by compact equipment with a high work efficiency.

BEST MODE OF CARRYING OUT THE INVENTION

The invention will be described in greater detail with reference to thefollowing reference example, examples and comparative example.

REFERENCE EXAMPLE 1

Crude 4,4'-compound was prepared by the process of U.S. Pat. No.4,162,270.

More specifically, a mixture of 290 g of phenol, 146 g of 98 wt. %sulfuric acid and 150 g of o-dichlorobenzene (ODCB) was heated withstirring. At about 150° C., the mixture started to boil and to distilloff ODCB and water formed on reaction. The distillate was condensed,with the ODCB phase continuously returned to the reaction system. Afterthe amount of the water layer reached 52 ml, the reaction temperaturewas maintained at 175 to 185° C., and the resulting water and ODCBcontaining a small amount of phenol were distilled off for recovery inan adjusted vacuum over a period of 4 hours until the reaction productwas solidified to dryness.

When analyzed by high performance liquid chromatography, the dryreaction product was found to contain 84.6 wt. % of 4,4'-compound, 2.0wt. % of 2,4'-compound, 4.5 wt. % of tri-compound and 8.9 wt. % of othersulfonic acids, etc.

EXAMPLE 1

A 100-gram portion of the dry reaction product, i.e. crude 4,4'-compoundobtained in Reference Example 1, was added to 155 g of water and 18.1 gof sodium hydroxide (the sum of the amount required for neutralizing thesulfonic acids and the amount corresponding to 1.2 times the equivalentbased on the total amount of the sulfones including 4,4'-compound,2,4'-compound and tri-compound). The mixture was heated to 95° C. withstirring. The product did not dissolve completely, forming a suspension.

With addition of 10 g of sodium chloride, the suspension was thenmaintained at the same temperature for 30 minutes for aging andthereafter cooled. After maintaining the mixture at 50° C. for 1 hour,the resulting precipitate was filtered off and washed with 50 ml of 5wt. % aqueous solution of sodium chloride, giving 84.0 g of monosodiumsalt of 4,4'-compound (purification yield 91.3%).

The monosodium salt of 4,4'-compound obtained was treated withhydrochloric acid and thereafter analyzed by high performance liquidchromatography, with the result given in Table 1.

The monosodium salt of 4,4'-compound obtained was dissolved in 900 ml ofwater and 78 wt. % of sulfuric acid was added to the solution at atemperature of 90 to 95° C. to adjust the solution to a pH of 4.0 andprecipitate 4,4'-compound. After cooling the reaction mixture to 40° C.,the precipitate was filtered off and dried, giving 76.6 g of purified4,4'-compound in a purification yield of 90.4% (based on the content of4,4'-compound in the crude).

Table 1 shows the result obtained by analyzing the purified product byhigh performance liquid chromatography.

EXAMPLE 2

The same salting-out step as in Example 1 was repeated with theexception of using 15 g of anhydrous sodium sulfate in place of 10 g ofsodium chloride serving as the salting-out agent in Example 1, giving83.2 g of monosodium salt of 4,4'-compound (purification yield 90.4%).The salt was similarly treated with sulfuric acid to obtain 75.9 g ofpurified 4,4'-compound.

The purification yield was 89.5%. The purified product was analyzed byhigh performance liquid chromatography, with the result shown in Table1.

EXAMPLE 3

A 100-gram portion of the dry reaction product, i.e. crude4,4'-compound, obtained in Reference Example 1, was added to 280 g ofwater and 31.6 g of sodium hydroxide (the sum of the amount required forneutralizing the sulfonic acids and the amount corresponding to 2.1times the equivalent based on the sulfones such as 4,4'-compound). Themixture was heated to 95° C. with stirring, whereby the product wascompletely dissolved.

Next, 24.1 g of 78 wt. % sulfuric acid was slowly added to the solution,which was then cooled to 50° C. and maintained at this temperature for 1hour. The crystals separating out were filtered off and washed with 5wt. % aqueous solution of sodium sulfate, giving 81.0 g of monosodiumsalt of 4,4'-compound (purification yield 88.0%).

The monosodium salt of 4,4'-compound obtained was analyzed by highperformance liquid chromatography. Table 1 shows the composition of thesalt thus determined.

The monosodium salt of 4,4'-compound obtained was treated with sulfuricacid in the same manner as in Example 1, affording 73.9 g of purified4,4'-compound. The purification yield was 87.3%. Table 1 shows theresult obtained by analyzing the product by high performance liquidchromatography.

EXAMPLE 4

A commercial crude product of 4,4'-compound was analyzed by highperformance liquid chromatography to find that the product contained82.6 wt. %, 14.0 wt. % and 3.4 wt. % of 4,4'-compound, 2,4'-compound andtri-compound, respectively.

The crude product (100 g) was added to 160 g of water and 33.2 g ofsodium hydroxide (2.1 times the equivalent based on the total amount ofthe sulfones such as 4,4'-compound), and the mixture was heated to 97°C. with stirring. The product did not dissolve completely, giving asuspension.

Next, 25.1 g of 78 wt. % sulfuric acid was slowly added to thesuspension, the mixture was cooled and maintained at 50° C. for 1 hour,and the crystals separating out were filtered off and washed with 5 wt.% aqueous solution of sodium sulfate to obtain 80.4 g of monosodium saltof 4,4'-compound (purification yield 87.4%).

The monosodium salt of 4,4'-compound obtained was analyzed by highperformance liquid chromatography. Table 1 shows the composition of thesalt thus determined.

The monosodium salt of 4,4'-compound obtained was treated in the samemanner as in Example 1, affording 71.5 g of purified 4,4'-compound in apurification yield of 85.8 wt. %. Table 1 shows the result obtained bysubjecting the purified product to high performance liquidchromatography.

                  TABLE 1                                                         ______________________________________                                                  4,4'-Compd.                                                                            2,4'-Compd. Tri-Compd.                                               (wt. %)  (wt. %)     (wt. %)                                        ______________________________________                                        Example 1                                                                     Monosodium salt                                                                           99.43      0.25        0.16                                       Purified product                                                                          99.88      0.09        0                                          Example 2                                                                     Monosodium salt                                                                           99.39      0.26        0.18                                       Purified product                                                                          99.87      0.09        0.01                                       Example 3                                                                     Monosodium salt                                                                           99.60      0.15        0.10                                       Purified product                                                                          99.92      0.05        0                                          Example 4                                                                     Monosodium salt                                                                           98.00      1.28        0.42                                       Purified product                                                                          99.08      0.65        0.05                                       ______________________________________                                    

EXAMPLE 5

A 100-gram portion of the dry reaction product, i.e. crude4,4'-compound, obtained in Reference Example 1, was added to 135 g ofwater and 27.5 g of ammonia water containing 28 wt. % of ammonia (thesum of the amount required for neutralizing the sulfonic acids and theamount corresponding to 1.2 times the equivalent based on the totalamount of the sulfones, i.e. 4,4'-compound, 2,4'-compound andtri-compound). The mixture was heated to 95° C. with stirring. Theproduct did not dissolve completely, forming a suspension.

With addition of 20 g of ammonium sulfate, the suspension was thenmaintained at the same temperature for 30 minutes for aging andthereafter cooled. After maintaining the mixture at 50° C. for 1 hour,the resulting precipitate was filtered off and washed with 50 ml of 5wt. % aqueous solution of ammonium sulfate, giving 80.8 g ofmonoammonium salt of 4,4'-compound (purification yield 89.4%).

The monoammonium salt of 4,4'-compound obtained was treated withhydrochloric acid and thereafter analyzed by high performance liquidchromatography, with the result listed in Table 2.

The monoammonium salt of 4,4'-compound obtained was dissolved in 900 mlof water, and 78 wt. % of sulfuric acid was added to the solution at atemperature of 90 to 95° C. to adjust the solution to a pH of 4.0 andprecipitate 4,4'-compound. After cooling the reaction mixture to 40° C.,the precipitate was filtered off and dried, affording 74.9 g of purified4,4'-compound in a purification yield of 88.5% (based on the content of4,4'-compound in the crude).

The product was analyzed by high performance liquid chromatography.Table 2 shows the result.

EXAMPLE 6

A 100-gram portion of the dry reaction product, i.e. crude4,4'-compound, obtained in Reference Example 1, was added to 200 g ofwater and 48.0 g of ammonia water containing 28 wt. % of ammonia (thesum of the amount required for neutralizing the sulfonic acids and theamount corresponding to 2.1 times the equivalent based on the sulfonessuch as 4,4'-compound). The mixture was heated to 95° C. with stirring.The product did not dissolve completely, forming a suspension.

Next, 24.1 g of 78 wt. % sulfuric acid was slowly added to thesuspension, the mixture was then cooled and maintained at 50° C. for 1hour, and the crystals separating out were filtered off and washed with50 ml of 5 wt. % aqueous solution of ammonium sulfate, affording 77.7 gof monoammonium salt of 4,4'-compound (purification yield 86.0%).

The monoammonium salt of 4,4'-compound obtained was analyzed by highperformance liquid chromatograhy. Table 2 shows the composition of thesalt thus determined.

The monoammonium salt of 4,4'-compound obtained was treated withsulfuric acid in the same manner as in Example 5, affording 72.1 g ofpurified 4,4'-compound in a purification yield of 85.2%. The purifiedproduct was analyzed by high performance liquid chromatography. Table 2shows the result.

                  TABLE 2                                                         ______________________________________                                                   4,4'-Compd.                                                                            2,4'-Compd.                                                                              Tri-Compd.                                                (wt. %)  (wt. %)    (wt. %)                                        ______________________________________                                        Example 5                                                                     Monoammonium salt                                                                          99.23      0.20       0.25                                       Purified product                                                                           99.87      0.04       0                                          Example 6                                                                     Monoammonium salt                                                                          99.49      0.12       0.18                                       Purified product                                                                           99.94      0.02       0                                          ______________________________________                                    

COMPARATIVE EXAMPLE 1

A 100-gram portion of the crude 4,4'-compound prepared in ReferenceExample 1, 31.6 g of sodium hydroxide and 280 g of water were stirred atan elevated temperature to prepare a solution, 48.0 g of 78 wt. %sulfuric acid was added to the solution at 95° C. to adjust the solutionto a pH of 4, and the mixture was cooled and maintained at 50° C. for 1hour. The resulting precipitate was filtered off and dried, giving 89.9g of 4,4'-compound in a yield of 99.3%. When analyzed by highperformance liquid chromatography, the product was found to contain 93.4wt. % of 4,4'-compound, 1.7 wt. % of 2,4'-compound and 4.9 wt. % oftri-compound.

We claim:
 1. A process for purifying crude 4,4'-dihydroxydiphenylsulfonecomprising adding said crude 4,4'-dihydroxydiphenylsulfone to an aqueoussolution of a basic substance whereby forming a salt of said sulfonewith the cation of said basic substance; adding a different soluble saltto said aqueous solution in an amount and under conditions wherebyprecipitating a mono-salt of 4,4'-dihydroxydiphenylsulfone from theresulting solution; separating the precipitated salt from the remainingaqueous phase; and subsequently treating the separated salt with an acidin an amount and under conditions whereby forming purified4,4'-dihydroxydiphenylsulfone.
 2. A process as defined in claim 1wherein the basic substance is a hydroxide or a carbonate of an alkalimetal or an alkaline earth metal, or ammonia.
 3. A process as defined inclaim 1 wherein the mono-salt of 4,4'-dihydroxydiphenylsulfone is amonoalkali metal salt, a mono 1/2 alkaline earth metal salt ormonoammonium salt thereof.
 4. A process as defined in claim 1 whereinsaid crude 4,4'-dihydroxydiphenylsulfone is dissolved in about 1.0 toabout 8.0 times its weight in water.
 5. A process for preparing amono-salt of 4,4'-dihydroxydiphenylsulfone comprising dissolving crude4,4'-dihydroxydiphenylsulfone in an aqueous solution of a basicsubstance; precipitating a mono-salt of 4,4'-dihydroxydiphenylsulfonefrom the resulting solution by salting out, and separating theprecipitated salt.
 6. A process as defined in claim 5 wherein the basicsubstance is a hydroxide or a carbonate of an alkali metal or analkaline earth metal, or ammonia.
 7. A process as defined in claim 5wherein the mono-salt of 4,4'-dihydroxydiphenylsulfone is a monoalkalimetal salt, mono 1/2 alkaline earth metal salt a or a monoammonium saltthereof.